MRI Web Clinic — June 2003

Posterolateral Corner InjuryMichael E. Stadnick, M.D.

Clinical History: 20 year-old male injured playing soccer. Sagittal proton-density weighted fat-suppressed images are provided. What are the findings? What is your diagnosis?

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Figure 1:

(1a, 1b, 1c) Sagittal proton-density weighted fat-suppressed images

Findings

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Figure 2:

(2a) The anterior cruciate ligament is completely torn, evidenced by laxity and hemorrhage(arrow).

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Figure 3:

(3a, 3b) Inspection of the posterolateral corner demonstrates hemorrhage in the popliteus muscle (3a arrow) and soft tissue hemorrhage and edema adjacent to the styloid process of the fibular head (3b arrow), at the site of arcuate ligament attachment. Bone bruises are present at the posterolateral tibia and within the fibular head (3a and 3b arrowheads).

Diagnosis

Discussion

Posterolateral stability of the knee is maintained by a complex and variable arrangement of ligaments and tendons known as the posterolateral corner. The arcuate complex, a component of the posterolateral corner, is composed of the arcuate ligament, the fibular collateral ligament, and the popliteus muscle. Additional elements of the posterolateral corner include the fabellofibular ligament, popliteofibular ligament, and the posterolateral capsule.

In patients with central ligamentous injuries, the presence of a posterolateral corner injury influences treatment and surgical planning. Untreated injuries to the posterolateral corner may lead to posterolateral knee instability and have been identified as a cause of anterior cruciate ligament graft failure1. When PCL injury and posterolateral instability coexist, surgical repair of the PCL is indicated. In such cases, extraarticular repair of the posterolateral corner is necessary to restore knee motion patterns2 and to improve the chances of success of the PCL reconstruction3.

Injuries to the posterolateral corner can occur as a result of excessive varus stress, severe external rotation injury of the tibia, and hyperextension injury. An isolated injury of the arcuate complex is uncommon. A coexisting cruciate ligament injury is typical and can make clinical evaluation of the posterolateral corner structures difficult. MRI readily identifies and assesses injuries of the posterolateral corner, alerting the orthopaedist to potential posterolateral instability.

Signs of posterolateral corner injury and potential posterolateral instability include fibular collateral ligament injuries, tears of the popliteus tendon or muscle, and hemorrhage and fluid posterior to the popliteus muscle or in the region of the arcuate ligament and posterolateral capsule. The “arcuate sign” is a fracture of the proximal fibula resulting from avulsive stresses by the biceps femoris and fibular collateral ligament insertions, and has a high incidence of associated injuries to the posterolateral capsule and cruciate ligaments.4

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Figure 5:

(5a) A coronal fat-suppressed T2-weighted image demonstrates the "arcuate sign", caused by an avulsion fracture of the fibula at the site of the fibular collateral ligament and biceps femoris tendon attachment (arrow). Associated injuries of the arcuate complex are demonstrated by extensive hemorrhage and edema within and posterior to the popliteus muscle (p), as seen on (5b) a fat-suppressed proton- density weighted axial view.

Conclusion

Injuries to the posterolateral corner are important to recognize but may be difficult to assess clinically because of coexisting injuries at the knee. In such cases, MRI can provide vital information regarding the status of the posterolateral corner, thus enabling more effective treatment and surgical planning.